Introduction


Figure 1. Pacific giant salamander (Credit: Sierra Nystrom)


Figure 2. The Mack Creek study site is located in the HJ Andrews Experimental Forest, Oregon. (Credit: Dana Warren)


Figure 3. Mack Creek, HJ Andrews Experimental Forest, OR (Credit: Oregon State University)

Data and Methods

Data for vertebrate numbers and size from 1987-2017 was provided by the Aquatic Vertebrate Population Study in Mack Creek, Andrews Experimental Forest, OR (Andrews Forest LTER). The two sub-reaches of Mack Creek sampled are in a section of clearcut forest (ca. 1963) and an upstream 500 year old coniferous forest. Sub-reaches are sampled with 2-pass electrofishing, and all captured vertebrates are measured and weighed. Beginning in 1993, the study began including sample data on Pacific giant salamanders, Dicamptodon tenebrosus, which is the core data set used in this analysis. For more information on collection methods and spatial information, see Metadata. This analysis used chi-squared testing to compare differences between salamander counts. Mean salamander weights in 2017 were compared using a two-sided t-test for two groups (old growth vs. clear cut reaches of Mack Creek), and one-way ANOVA with post-hoc Tukey’s when comparing >2 groups (channel classification); α = 0.05 was used throughout. Effect sizes were calculated using Cohen’s d. All analyses and figures were prepared using R software version 3.6.1.

Results

Figure 4. Number of Pacific giant salamanders recorded in Mack Creek from 1993-2017. Salamander counts have been seperated by whether they were recorded in the section of creek that runs through old growth forest or forest that was clear cut. Data: Andrews Forest LTER

Figure 5. Number of Pacific giant salamanders recorded in Mack Creek in 2017 by creek section (OG/CC) and channel classification.
Channel Class Clear Cut Old Growth
Cascade 247 (55%) 201 (45%)
Pool 31 (41%) 45 (59%)
Side Channel 90 (55%) 74 (45%)

There is not a significant association between forest condition (old growth/clear cut) on where in the channel salamanders are found (pool/side-channel/cascade) for salamanders sampled in 2017. (\(\chi\)2(2) = 5.54, p = 0.063).

Figure 6. Recorded weights of Pacific giant salamanders sampled in Mack Creek in 2017 by section of the creek found (CC/OG). The black dot represents the sample mean with error bars representing the range of uncertainty. Data: Andrews Forest LTER

Figure 7. Pacific giant salamander sample statistics in 2017 by section of Mack Creek (CC/OG)
Creek Section Mean Weight (g) Standard Deviation (g) Sample Size Standard error Variance
Clear Cut 7.78 9.90 368 0.52 98.1
Old Growth 6.58 8.96 328 0.49 80.2

There is not enough evidence to conclude that mean Pacific giant salamander size in the clear cut section of Mack Creek in 2017 (7.78 \(\pm\) 9.9, n = TRUE, 368) differed significantly from mean Pacific giant salamander size in the old growth section of Mack Creek in 2017 (6.58 \(\pm\) 8.96, n = TRUE, 328) by a two-sided, two sample t-test (t(692.79) = -1.67, p = 0.096). In addition, the effect size between mean sizes is negligible (Cohen’s d = 0.13).

Figure 8. Recorded weights of Pacific giant salamanders sampled in Mack Creek in 2017 by channel classification. Data: Andrews Forest LTER

Figure 9. Using a quantile-quantile plot we can assess whther the 2017 salamander sample comes from a population that has weights which are normally distributed. We do not see the linear trend that would suggest normality for all three channel classifications. Data: Andrews Forest LTER

Figure 10. Recorded weights of Pacific giant salamanders sampled in Mack Creek in 2017 by channel classification. The black dot represents the sample mean with error bars representing the range of uncertainty.

Figure 11. Pacific giant salamander sample statistics in 2017 by different channel classifications in Mack Creek
Channel Class Mean Weight (g) Standard Deviation (g) Sample Size Standard error Variance
Cascade 7.52 9.03 448 0.43 81.50
Pool 9.30 13.62 76 1.56 185.58
Side Channel 5.68 8.27 164 0.65 68.31

Mean salamander weight (g) differed significantly between the three creek channel classifications (pool, cascade, side-channel) by a one-way ANOVA test (F(2, 684) = 4.22, p = 0.015). Further pairwise testing using pot-hoc Tukey’s HSD revealed that mean salamander weight was only significantly different between side-channel and pool classifications (p = 0.017). Though a significant difference in mean weight was found between salamanders sampled in the pools vs. side-channel sections of Mack Creek in 2017, the effect size was small (Cohen’s d = 0.21).

Summary

References

Gregory S. V. 2016. Aquatic Vertebrate Population Study in Mack Creek, Andrews Experimental Forest, 1987 to present. Environmental Data Initiative. https://doi.org/10.6073/pasta/5de64af9c11579266ef20da2ff32f702. Dataset accessed 11/27/2019.